1. Field of the Invention
The present invention relates to superparamagnetic core shell nanoparticles having an iron oxide core, a silica shell and a metal silicate interface layer between the iron oxide core and silica shell and a magnetic core produced with these nanoparticles. The core of the present invention is suitable for utility in power generation parts such as stators, rotors, armatures and actuators or any device whose function is dependent upon an efficient magnetic core, i.e., a magnetic core having a high magnetic moment, minimal magnetic hysteresis and no or little eddy current formation.
2. Discussion of the Background
Many electronic devices rely on magnetic cores as a method of transferring a magnetic field. Due to inefficiency caused by core loss, a portion of this power is lost, typically as waste heat. A core's magnetic properties have the ability to greatly concentrate and enhance magnetic fields. Thus, improving and implementing core materials with low loss as well as high magnetic permeability would enormously enhance the efficiency of the device. With increased interest in environmentally-conscious devices, the implementation of improved magnetic core material across millions and millions of devices that require them (all computers, TVs, cell phones, vehicle power electronics, etc.) could produce significant benefits for global energy conservation.
Magnetic materials generally fall into two classes which are designated as magnetically hard substances which may be permanently magnetized or soft magnetic materials which may be reversed in magnetism at low applied fields. It is important in soft magnetic materials that energy loss, normally referenced as “core loss” is kept to a minimum whereas in hard magnetic materials it is preferred to resist changes in magnetization. High core losses are therefore characteristic of permanent magnetic materials and are undesirable in soft magnetic materials.
The use of powdered magnetic materials allows the manufacture of magnetic parts having a wide variety of shapes and sizes. However, materials made from consolidated powdered magnetic materials have been limited to utility in applications involving direct currents. Direct current applications, unlike alternating current applications, do not require that the magnetic particles be insulated from one another in order to reduce eddy currents.
Conventionally, magnetic device parts are constructed from powders by compaction of the powders to a defined shape and then sintering the compact at temperatures of 600° C. or higher. Sintering the part following compaction, is necessary to achieve satisfactory mechanical properties in the part by providing particle to particle bonding and hence strength. However, sintering may cause volume changes and results in a manufacturing process with poor dimensional control.
In other processes designed to prepare parts having minimum eddy current losses, the magnetic particles are coated with thermoplastic materials before pressing. The plastic is provided to act as a barrier between the particles to reduce induced eddy current losses. However, in addition to the relatively high cost of such coatings, the plastic has poor mechanical strength and as a result, parts made using plastic-coated particles have relatively low mechanical strength. Additionally, many of these plastic-coated powders require a high level of binder when pressed. This results in decreased density of the pressed core part and, consequently, a decrease in magnetic permeability and lower induction. Additionally, and significantly, such plastic coatings typically degrade at temperatures of 150-200° C. Accordingly, magnetic parts made in such manner are generally limited to utility in low stress applications for which dimensional control is not critical.
Soft magnetic core components are frequently used in electrical/magnetic conversion devices such as motors, generators and transformers and alternators, particularly those found in automobile engines. The most important characteristics of soft magnetic core components are their maximum induction, magnetic permeability, and core loss characteristics. When a magnetic material is exposed to a rapidly varying magnetic field, a resultant energy loss in the core material occurs. These core losses are commonly divided into two principle contributing phenomena: hysteresis and eddy current losses. Hysteresis loss results from the expenditure of energy to overcome the retained magnetic forces within the core component. Eddy current loss, the other source of core loss, refers to circular currents setup within the magnetic core due to the applied magnetic field, as explained by Faraday's Law. Eddy current losses are brought about by the production of induced currents in the core component due to the changing flux caused by alternating current (AC) conditions. These circular currents create a magnetic field anti-parallel to the applied field, decreasing the overall field within the core. In order to reduce eddy current formation, materials with low electrical conductivities are used.
Magnetic core inefficiency is measured in terms of core loss. To improve core loss, the magnetic core must demonstrate a reduced measure of magnetic hysteresis as well as lowered eddy current formation.
Another important magnetic property of materials for soft magnetic components is the magnetic saturation (Ms) of the material. Magnetic saturation is empirically measured and is representative of the total magnetic moment of a material sample. A low Ms can limit the application utility of a material and therefore, a high Ms is an important property to be an effective and useful magnetic material.
The magnetic saturation is influenced by a number of factors, which includes material composition, crystallinity and the stress-strain exerted on the material during production.
Applicants have described a magnetic core of significantly reduced magnetic hysteresis and low eddy current formation obtained by sintering superparamagnetic core shell nanoparticles having an iron oxide core and silica shell into a monolithic core structure in U.S. application Ser. No. 13/529,316, filed Jun. 21, 2012, issued as U.S. Pat. No. 9,067,833 on Jun. 30, 3015, the disclosure of which is incorporated herein by reference in its entirety. A core of high magnetic saturation based on core shell nanoparticles having an iron oxide core and silica shell was described in U.S. application Ser. No. 13/900,860, filed May 23, 2013, issued as U.S. Pat. No. 9,093,205 on Jul. 28, 2015, the disclosure of which is incorporated herein by reference in its entirety.
However, there remains a need for magnetic powders having controllable or tunable magnetic properties which allow for production of tailored soft magnetic parts, having green strength, high temperature tolerance, and good mechanical properties, which provide parts having minimal or essentially no core loss and high magnetic moment.
Conventionally, ferromagnetic powders have been employed for the production of soft magnetic core devices. Such powders are generally in a size range measured in microns and are obtained by a mechanical milling diminution of a bulk material. Superparamagnetic nanoparticle materials having particle size of less than 100 nm have found utility for magnetic record imaging, as probes for medical imaging and have been applied for targeted delivery of therapeutic agents. However, the utilization of superparamagnetic powders of nanoparticles for production of core magnetic parts has until now, been limited.
Herzog et al. (U.S. 2012/0130023) describes a heat activated initiator composition for cross linking and curing polymers. In addition to a peroxide or compound having a labile carbon-carbon bond, the composition contains core shell magnetic particles. The core may be iron oxide or other magnetic material such as Fe, Co, Ni or an alloy such as FePt. The particle size of the core is 5-100 nm. The shell is a silica shell. However, multiple shells are possible and a metal silicate layer near the core is described. These core shell nanoparticles are produced by flame pyrolysis and nowhere is “tuning” the magnetic properties of the nanoparticles disclosed. Moreover, nowhere is formation of a magnetic core described.
Katusic et al. (U.S. 2011/0207869) describes core shell nanoparticles produced by flame pyrolysis. The core is an iron oxide and the shell is a silicon oxide. A bonding layer containing an iron silicate is present between the core and the shell. As these core shell nanoparticles are produced by flame pyrolysis “tuning” the magnetic properties of the nanoparticles is not disclosed. Moreover, nowhere is formation of a magnetic core described.
Liu (U.S. 2010/0054981) describes a bulk nanocomposite permanent magnet having a combination of hard phases and soft phases intermixed. CoFe is included as an example of a soft magnetic alloy. Synthesis of FePt nanoparticles is described and coating of the FePt nanoparticle with Fe3O4 is also described. However, no core shell structures of a soft phase particle coated with an insulative silica shell is disclosed or suggested.
Blagev (U.S. Pat. No. 5,512,317) describes coated iron oxide particles having a coating of a cobalt doped iron oxide and a silicate. The particles are not nano-sized and are of the order of 0.1 to 0.4 μm. However, this reference does describe a type of metal silicate interface layer on the iron oxide particles. “Tuning” the magnetic properties of the particles by control of the thickness of metal silica interface is not disclosed.
Soileau et al. (U.S. Pat. No. 4,601,765) describes a coated iron powder which is compacted to a magnetic core. The powder is less than 0.05″ in particle size and therefore not a nanoparticle according to the present invention. This reference does describe a combination of silicate coating and silicon coating on the powder but does not disclose or suggest “tuning” the magnetic properties of the particles and formed core by control of the thickness of the metal silicate interface.
Sun et al (J. Am. Chem. Soc., 2002, 124, 8204-8205) describes a method to produce monodisperse magnetite nanoparticles which can be employed as seeds to grow larger nanoparticles of up to 20 nm in size. Silica coating and metal silicate interface is not disclosed by this reference.
Bumb et al. (Nanotechnology, 19, 2008, 335601) describes synthesis of superparamagnetic iron oxide nanoparticles of 10-40 nm encapsulated in a silica coating layer of approximately 2 nm. Utility in power transformers is referenced, but no description of preparation of core structures is provided. “Tuning” the magnetic properties of the particles by control of the thickness of metal silica interface is not disclosed.
Zhang et al. (Nanotechnology, 19, 2008, 085601) describes synthesis of silica coated iron oxide particles. The average size of the iron oxide particle to be coated is 8 to 10 nm and the silica core is about 2 nm. “Tuning” the magnetic properties of the particles by control of the thickness of metal silica interface is not disclosed.
Tajima et al. (IEEE Transactions on Magnetics, Vol. 41, No. 10, October, 2005) describes a method to produce a powder magnetic core described as warm compaction using die wall lubrication (WC-DWL). According to the method an iron powder coated with a phosphate insulator was compacted under a pressure of 1176 MPa at a temperature of 423K to produce a core type structure.
Park et al. (J. Phys. Condens. Matter. 20, 2008, 204105) describes the synthesis of core-shell nanoparticles (described as temperature-sensitive ferrite (TSF) covered with silica). The ferrite core (mean diameter of 10 nm) was prepared by coprecipitation in an alkaline solution. Then, silica coating was carried out by the controlled hydrolysis and condensation of tetraethyl orthosilicate (TEOS). The core-shell particles were formed by a surface precipitation procedure using TSF nanoparticles as a core material. The thickness of the silica layer on TSF cores was observed by means of transmission electron microscopy (TEM). However, “tuning” the magnetic properties of the particles by control of the thickness of metal silica interface is not disclosed.
Yu et al. (J. Phys. Chem. C 2009, 113, 537-543) describes the preparation of magnetic iron oxide nanoparticles encapsulated in a silica shell. Utility of the particles as magnetic binding agents for proteins is studied.
None of the above references recognizes the significance of a metal silicate interface which is formed between the core and silica shell. None of these references disclose or suggest that core shell nanoparticles having an iron oxide core and silica shell with an iron silicate interface can be tuned with respect to magnetic properties by control of the thickness of the iron silicate layer. Likewise, none of the above references disclose or suggest a monolithic magnetic core constructed by heated compression of nanoparticular iron oxide encapsulated in a silica shell having an iron silicate interface layer of controlled thickness, wherein the particles are directly compacted without addition of lubricant or other material to facilitate particle adherence.
An object of the present invention is to provide a magnetic powder to produce soft magnetic parts, having in addition to increased green strength, high temperature tolerance, good mechanical properties, minimal or essentially no core loss, high magnetic saturation and a property of being tunable in terms of magnetic properties.
A second object of the invention is to provide a magnetic core having a high total magnetic moment and little or no core loss wherein the magnetic properties can be altered or controlled according to the tunable properties of the core shell nanoparticles.
A third object is to provide a method to produce a magnetic core or shaped core part having a high total magnetic moment and little or no core loss.